SDL Regex Fuzzer is a tool to help test regular expressions for potential denial of service vulnerabilities also known as ReDos attack.
SDL Regex Fuzzer uses the .NET traditional NFA regex engine to perform its analysis.

What is ReDos attack?

ReDos (Regular ExpressionDenial of service) is the attack carried out to make system unresponsive or unavailable to the desired user(s).It is the result of poorly coded or handled regular expressions.
Some of the regular expressions that can be used in a dos attack are listed below.

(a+)+

([a-zA-Z]+)*

(a|aa)+

(a|a?)+

(.*a){x} | for x > 10

The entire above are susceptible to the input aaaaaaaaaaaaaaaaaaaaaaaa! (The minimum input length might change slightly, when using faster or slower machines).